The present invention relates generally to objective lens driving devices and, more particularly, to an objective lens driving device using a multipole magnet to effect both tracking and focusing in an optical pick-up.
A conventional objective lens driving device is shown in FIGS. 29 through 31. An objective lens 1 and a counter weight 2 are mounted on opposite ends of an elliptic lens holder 3. The lens holder 3 has a tubular bearing 3a and a coil holder 3m disposed concentrically on the underside. A supporting shaft 4 is inserted in the tubular bearing 3a to support the lens holder 3 in such a manner that the lens holder 3 may move both in a vertical direction as shown by an arrow A in FIG. 30 and in a rotational direction as shown by an arrow B in FIG. 31. An elastic holder 5 is used to hold the lens holder 3 to permit such bi-directional movement of the lens holder 3.
A pair of permanent magnets 6a and 6b are disposed so that the upper ends (and lower ends) of the respective magnets have opposite polarities. A pair of yokes 7a and 7b are placed on top of the permanent magnets 6a and 6b. A disk-shaped yoke or holding base 8a supports the supporting shaft 4 and the permanent magnets 6a and 6b. A tubular central yoke 8c is made integrally with the disk-shaped yoke 8a.
The elastic holder 5 is secured to a rectangular base 9 with a pair of screws 12a and 12b. A focusing coil 10 is mounted on the underside of the lens holder 3. Four tracking coils 11a, 11b, 11c, and 11d are attached to the side of the focusing coil 10 at equal intervals. A cover 13 is joined to the holding base 9 at four corners to complete a movable lens assembly.
The bearing 3a is disposed inside the central yoke 8c, while the coil support 3b made integral with the coil holder 3m is disposed outside the central yoke 8c. The focusing coil 10 is wound around the cylindrical coil support 3b, and the tracking coils 11a, 11b, 11c, and 11d are fixed to the side of the focusing coil 10 so as to cover part of the outside thereof. Each tracking coil 11a-11d is wound in a rectangular shape to have a pair of vertical sides 11ab and 11af, 11bf and 11bb, 11cb and 11cf, or 11df and 11db, each extending in parallel to the supporting shaft 4. These coils and permanent magnets produce a force for moving the lens holder with the objective lens.
In operation, by flowing a certain amount of electric current through the focusing coil 10, the lens holder 3 is moved in the vertical direction shown by the arrow A in FIG. 30 to focus the objective lens 1. When electric currents are flown through the tracking coils 11a-11d, forces acting upon the vertical coil sides 11af, 11bf, 11cf, and 11df rotates the lens holder 3 with the objective lens 1 in either direction, depending on the direction of the current, shown by an arrow B in FIG. 31. In this way, tracking control is carried out.
In the conventional objective lens driving device such as described above, the focusing coil 10 and the tracking coils 11a-11d use the same magnetic circuit made up of the permanent magnets 6a and 6b and the yokes 7a, 7b, 8a and 8c. The permanent magnets 6a and 6b produce magnetic fluxes running in one direction; e.g., from the yokes 7a and 7b to the central yoke 8c so that forces act on only one vertical coil side 11af, 11bf, 11cf, or 11df of each tracking coil 11a-11d to move the lens holder 3 in the direction shown by the arrow B. In other words, both the vertical coil sides are not used to move the lens holder 3 in the direction of the arrow B because the magnetic fluxes acting on both the vertical coil sides 11af and 11ab, 11bf and 11bb, 11cb and 11cf, and 11 df and 11db have the same direction, thus reducing the efficiency of the tracking coils 11a-11d.
Consequently, this low efficiency makes it necessary to increase the number of windings and the coil size in order to provide the required torque. The large coils, however, not only fails to meet a recent demand for miniature coils but also expensive.